Do you intend for the user to be able to manage plugins after installing the app? Or do you just want to use plugins for making it easy for you, the developer, to add features later.

In the latter situation, you could get away with putting the plugins in your bundle.

But in the more likely situation, where you want the user to be able to add third party plugins, you should not, under any circumstances, store them as part of your app bundle. This was always a bad practice, but will cause your app to misbehave under Leopard if the user is using a firewall, and generally harass them with warnings about your app being an "Application downloaded from the Internet" every time they add or remove a plugin.

This means you need to put those files in the ApplicationSupport/YourApp folder, which is really where they belong anyway. Regardless of where you put them, you're going to need to manage dynamic library loading to get the code in, and that's not something I'm that familiar with in OSX-land.